The present invention relates to the field of anti-counterfeiting and authentication methods and devices and, more particularly, to methods, security devices and apparatuses for authenticating documents and valuable products by color fluorescent images invisible or barely visible under day light.
Since high-quality and low-priced color photocopiers and desk-top publishing systems are available, counterfeiting of documents is becoming now more than ever a serious problem. The same is also true for other valuable products such as CDs, DVDs, software packages, medical drugs, watches, etc., that are often marketed in easy to falsify packages.
The present invention provides a novel security element offering enhanced security for devices needing to be protected against counterfeits, such as banknotes, checks, credit cards, identity cards, travel documents, valuable business documents, and packages of goods such medical drugs.
A further application concerns valuable products where protective and decorative features can be combined. For example luxury goods such as watches and clocks, bottles of expensive liquids (perfumes, body care liquids, alcoholic drinks), clothes (e.g. dresses, skirts, blouses, jackets and pants), may exhibit striking fluorescent color images when viewed under UV light and at the same time prevent counterfeits by making the unauthorized reproduction of such fluorescent color images very difficult to achieve.
As a further application field, the present invention also enables creating digital fluorescent color images for commercial art, decoration, publicity displays, fashion articles, and night life, where fluorescent images viewed under UV illumination at night or in the dark have a strongly appealing effect.
Since a long time, fluorescent inks visible under UV light but invisible under normal day light are used for the authentication of security documents, such as passports, bank notes, checks and vouchers, see Van Renesse, R. L., 2005, Optical Document Security, Artech House, London, England, pp. 97-102. A single ink layer is used for printing either text or a bilevel image. Fluorescent inks are extensively used in the Euro bank notes, where both the stars and the silhouette of Europe are highlighted under UV light. However, since fluorescent inks are available on the market, their protection against counterfeits have decreased.
A recent challenge consists in trying to create color images by using several fluorescent inks each emitting in a different part of the visible wavelength range. U.S. Pat. No. 7,054,038, Method and apparatus for generating digital halftone images by multi color dithering, filed Jan. 4, 2000, to Ostromoukhov and Hersch (also inventor in the present patent application), teaches a multi-color dithering method where one or more inks are possibly fluorescent inks. However, they do not explicitly take into account the quenching effect and do not provide solutions for reducing the concentration of the ink by printing smaller ink dots.
Patent application Ser. No. 10/818,058, “Methods and ink compositions for invisibly printed security images having multiple authentication features”, to Coyle, W. J. and Smith, J. C, filed Apr. 5, 2004, proposes to create fluorescent color images with red, green and blue emitting fluorescent inks, which are invisible under day light. They advocate to perform the color separation from classical cyan, magenta and yellow to red, green and blue fluorescent inks by converting the image colors to their negative form using commercially available computer software such as Adobe PhotoShop. That disclosure converts an image color to its negative form by starting with an input cyan (c), magenta (m) and yellow (y) image, and deducing the corresponding surface coverages of red (r), green (g) and blue (b) by simple negation, i.e. r=1−c, g=1−m, b=1−y. By replacing cyan, magenta and yellow cartridge inks with red, green and blue fluorescent ink cartridges and by printing the three ink layers in mutual registration, a color fluorescent image is obtained. That patent application proposes to use the halftoning of standard ink-jet printers, generally error-diffusion or blue noise dithering. These halftoning methods rely on the superposition of the ink layers and may therefore yield quenching effects reducing the fluorescent emission spectra. That patent application also does not teach how to expand the fluorescent color gamut by creating additional colorants through the superposition of fluorescent ink dots, possibly at a reduced dot size. In addition, since red, green and blue fluorescent inks are starting to become available on the market, and since converting an image to its negative by commercially available computer software is accessible to the public, the protection offered by this method may become limited.
U.S. Pat. No. 7,005,166 B2, Method for fluorescent image formation, print produced thereby and thermal transfer sheet thereof, to Narita and Eto (2002), teaches how to thermally transfer red, green and blue fluorescent dyes onto paper, thereby forming an image with color gradations. They neither propose an explicit halftoning method nor do they deal with the problem of reproducing an input color image.